The invention relates generally to overvoltage protection equipment for electrical apparatus and particularly to protection equipment for series capacitor installations for high voltage alternating current transmission lines.
Capacitor banks are connected in series in high voltage transmission lines to compensate a desired part of the inductance of the line in order to stabilize the system, control load division between parallel lines, or provide other benefits. Since the capacitors are in series in the line they are subject to dangerous overvoltages in case of a fault on the line or other excess current conditions such as may result from switching surges.
A substantial body of technology has developed for the purpose of protecting series capacitor banks against harmful faults. Normally the general nature of the protection system is to provide a bypass around the capacitor bank, such as by breakdown of a parallel spark gap, when a harmful condition occurs. This of course means that during the period of time the capacitor is being bypassed by a direct conductive path, it is not contributing capacitance to the system and the intended benefits of the capacitor installation are not provided. Consequently, it is very desirable that the protective equipment be sufficient to protect the capacitors against conditions that would damage them but that any over protection be minimized by not completely removing the series capacitor during brief overvoltages. If the bypass equipment operates on such a transient, the problem then becomes how to restore the capacitor to the transmission line quickly and efficiently. This can be a difficult problem because transients normally occurring immediately upon reinsertion of the capacitor in the transmission line would result in danger of repeated operation of the bypass equipment.
This background on problems of series capacitor protection equipment and their solution is further discussed in commonly assigned copending application Ser. No. 500,996, filed Aug. 27, 1974 by C. A. Peterson , now U.S. Pat. No. 3,889,158, issued June 10, 1975, and Grove et al U.S. Pat. No. 3,801,870, issued Apr. 2, 1974, and the additional patents referred to therein.
Among electrical components are those known as nonlinear resistors that exhibit a distinct change in resistance magnitude upon occurrence of certain conditions. A familiar type is that referred to as a carbon pile that comprises a plurality of carbon plates stacked in sequence in an operating circuit with provision for varying the contact pressure between the plates. The amount of resistance exhibited by the carbon pile decreases as the pressure increases because of improvement of the interface contact. Certain forms of such devices are arranged so that upon occurrence of a predetermined current flow through the carbon pile there is an abrupt release of pressure on the plates with the result that there is an abrupt increase in resistance serving to limit the current through the pile to a predetermined value. The abrupt increase in resistance in some applications is intended to be sufficiently high that the pile acts as a switch opening the circuit. For background on such devices reference is made to Whittaker U.S. Pat. No. 2,406,449, Aug. 27, 1946, and Hicks et al U.S. Pat. No. 3,764,851, Oct. 9, 1973. Such pressure dependent current limiting devices are subject to variation in performance with continued use because of wear caused either mechanically or by electrical discharges at the interfaces.
Another known type of carbon pile device that has been applied in practice is that sometimes referred to as an "auto valve" arrester for electrical surge suppression such as in lightning arresters. In this device the plates, such as of carbon, are spaced a small distance, such as 4 or 5 mils, by insulating spacers. Such carbon piles with intermediate spacing exhibit a relatively high characteristic voltage such as about 350 volts at which breakdown by glow discharge occurs with relatively low current conduction capability.
It has been previously recognized, such as in the work relating to the current limiting device of above-mentioned U.S. Pat. No. 3,764,851, that even in direct contact carbon piles the extent to which arcing occurs and the nature of that arcing influences the characteristics of the device. It has been recognized, for example, that where the surfaces are sufficiently rough and the applied pressure is such that the plate contact is limited to a number of spaced points or areas that an arc discharge occurs, as opposed to a glow discharge, in which state the voltage across the pile is limited to a characteristic value but the range of current that can be conducted in that state is wide. For example, under these conditions a typical pile may exhibit a "short arc voltage" of approximately 20 volts over a current range of from about 100 amperes to thousands of amperes.